Laser interferometric studies of solder joints of surface mounted chips

Abstract

In order to understand the stress-strain relationship in surface mount solder joints. previous research workers used strain gauges to measure the deformation of these joints. However, this method is indirect and not very accurate. In this Ph.D. project, a non-contact and highly accurate method based on laser Interferometry has been specially developed to measure displacements in solder joints and to obtain an understanding of the deformation behaviour of these joints. The laser interferometric system is modified from a Mach Zehnder interferometer. The system is applied to measure the deformation of surface mount solder joints under simulated operating conditions. A simple model using the lumped capacitance method is proposed to describe the history of the measured deformations. By using the interferometric system, the surface deformations of several idealized surface mounted solder joints as well as other surfaces in the specimens have been measured. Typical deformations are found to be in the micrometer range. The average stress in the joints is then deduced from the measured deformations and comparisons of the deduced stress are made among the specimens. The experimental results are further compared with the results obtained by Finite Element Analysis (FEA). This shows that experimental and calculated results are in good agreement.